Apparatus for producing wood-plastic composites

ABSTRACT

A system for converting wood to a wood-plastic composite in which the steps of pressure impregnation of the wood with a monomer and polymerization of the monomer within the wood by either heat or irradiation is accomplished in a single container. The system includes an apparatus by which a thin walled container is able to withstand the internal pressure of monomer impregnation and a method and apparatus for handling the container in the polymerization processes. The system further includes an irradiation tank equipped with an irradiation absorbing fluid recirculation system for bacteria control and an inert gas system for displacing the irradiation absorbing fluid from between the submerged containers and the irradiation source. The irradiation source includes a plurality of line sources variable in spacing and in the intensity of irradiation. Means are provided to overcome the spatial variation in radiation produced by the discrete line sources by a time oscillation of the array of line sources with respect to the container.

United States Patent Barrett 1 Feb. 29, 1972 [72] Inventor:

Primary ExaminerMorris Kaplan Attorney-Bums, Doane, Swecker & Mathis Lawrence G. Barrett, Lynchburg, Va. 57] ABSTRACT Assigneei The American Novawood m A system for converting wood to a wood-plastic composite in Lynchburg, which the steps of pressure impregnation of the wood with a [22] Filed: och 27, 1967 monomer and polymerization of the monomer within the wood by either heat or irradiation 1s accomplished in a single [2]] Appl. No.: 678,584 container.

The system includes an apparatus by which a thin walled con- 9, ll8/503, tainer is able to withstand the internal pressure of monomer 250/52 impregnation and a method and apparatus for handling the [51] Int. Cl ..C23c 13/08 container in the polymerization processes. [58] Field of Search ..118/50, 50.1, 620, 641-643;

The system further includes an irradiation tank equipped with 198/209, 131

an lrradlatlon absorbing fluid recirculation system for bacteria 56 R f control and an inert gas system for displacing the irradiation l e erences cued absorbing fluid from between the submerged containers and UNITED STATES PATENTS the irradiation source. The irradiation source includes a plurality of line sources variable in spacing and in the intensity of 1,986,319 l/ l 935 Bongrand et al ..1 18/50 irradiation Means are provided to overcome the Spatial 2,858,795 11/1958 walker 8/50 tion in radiation produced by the discrete line sources by a 2,910,959 1 H1959 f 118/50 time oscillation of the array of line sources with respect to the 3,233,579 2/1966 Arvidsson 118/50 containen 3,448,719 6/1969 Tate 118/50 20 Claims, 12 Drawing Figures WOOD CANISTER PREPARATION LOAD'NG PREPARATION IMPREGNATING I. VACUUM 2- INERT GAS 3- VACUUM 4-IMPREGNATING FLUID 5 INERT GAS POLYMERIZING HEAT 0R IRRA DIATION PRODUCT L0 mg FINISHING UN A PATENTEI] FEB 2 9 I972 WOOD PREPARATION sum 1 or 5 PRODUCT FINISHING LOADING CANISTER IMPREGNATING I- VACUUM 2 INERT GAS 3- VACUUM 4-IMPREGNATING FLUID 5 IN ERT GAS POLYMERIZING HEAT 0R IRRADIATION UNLOADING PREPA RATION FIGI LAWRENCE G. BARRETT BYMQIM/WI ATTORNEYS PATENTEDFEB 29 I972 SHEET 2 [1F 5 FIG. 5a

INVENTOR LAWRENCE G BARRETT ATTORNEYS PATENTEUFEBZQ m2 SHEET 3 BF 5 INVENTOR LAWRENCE e BARRETT ATTOR NEYS' PATENTEDrwzsnsrz SHEET 0F 5 STEAM LAWRENCE G. BARRETT ATTORNEYS PATENTEDFEB 29 I972 SHEET 5 BF 5 FIGIO FIGII INVENTOR BARRETT LAWRENCE BY M @0444, M4 Am4r wad;

ATTORNEYS APPARATUS F OR PRODUCING WOOD-PLASTIC COMPOSITES BACKGROUND OF THE INVENTION This invention relates to an apparatus for manufacturing wood-plastic composites and more particularly to a method and apparatus for the economic production of high-quality wood-plastic composites with increased protection for the safety of operating personnel and a reduction of potential hazards in handling of the materials.

The basic process of manufacturing wood-plastic composites is well known. It involves the steps of impregnating the wood with a monomer and curing to effect the polymerization of the impregnated monomer either by irradiating the impregnated wood with gamma rays or by the use of heat. The characteristics of the resulting product are controlled by varying the amount and depth of monomer impregnation into the wood, the choice of monomer, the viscosity of the impregnating monomer which is varied through the addition of prepolymerized polymer, and the inclusion of additives tending to improve selected characteristics such as fire retardancy, fungus resistance, heat distortion temperature and chemical resistance. In addition, the selection of technique for polymerization such as by radiation or heat curing will influence the characteristics of the resulting product. The principal differences in the finished product result from the lower temperatures involved in radiation polymerization and also the higher degree of cross-linking.

The first of these steps, the impregnation of the wood with the monomer, has heretofore been carried out by techniques well known and used in the treatment of wood with preservatives and fire retardants. The impregnation step is desirably accomplished under positive pressure to facilitate the loading of the wood with the liquid monomer. Carbon steel has been the conventional and obvious choice for the material from which the pressure vessel is constructed based on the twin virtues of strength and relative economy. By the adjustment of the time and amount of pressure used in the impregnation step and the entrapment of an inert gas within the wood, the loading and the distribution of the monomer within the wood can be controlled.

The second step, that of irradiating the monomer impregnated wood to effect polymerization thereof, must also be carried out in a vaportight container as the vapor pressure of the monomer is quite high and a substantial monomer loss would be incurred during the irradiation period to the detriment of the desired loading parameters. Vaporization of the monomer would also expose the operating personnel to a significant health hazard as the fumes are quite toxic. The vapor is also inflammable and the evaporation thereof creates a further hazard in the form of a potential explosive atmosphere. Additionally, the use of an inert gas during the polymerization process to prevent oxygen inhibition has been found extremely desirable.

The walls of the irradiation vessels are desirably constructed of low-density materials of a minimum thickness to minimize the parasitic absorption of the curing radiation energy such as gamma rays. Further, in order to obtain a high efficiency of radioactive source utilization, the geometry of the irradiation vessels should substantially conform to the geometry of the product. The practical design considerations for radioactive source elements also lead to planar source construction. This stems from the need of encapsulation of the basic radioactive source materials, efficient production of the radioactive source materials in nuclear reactors, and the need for remote assembly of the source elements into composite source assemblies. As the products typically are in the form of flat sheets, geometrical conformity suggests that the sides of the irradiation container should also be planar.

The incompatibility of the pressure vessels with their curved sides of heavy carbon steel and the irradiation vessels with their flat surfaces of low-density material has heretofore necessitated the transfer of the impregnated wood from the pressure vessel to an irradiation vessel. Monomer is unavoidably lost during this transfer period through vaporization and the operating personnel are subjected to the potential hazards of a toxic and explosive atmosphere. The loss of monomer is of course not only expensive but detrimental to the quality of the product.

Accordingly, it is one object of this invention to provide an apparatus whereby the same vessel can be utilized for the steps of impregnation and curing of the impregnated product.

Another object is to provide an apparatus whereby quick access is obtained to the vessel's interior for loading and unloading.

Still another object is to provide a novel, thin-walled container of low-density material having large, flat sides that is adapted allowing the efficient use of a radiation source and yet can be utilized as a vessel for impregnating wood under pressure and will permit a vacuum to be drawn within the impregnation-polymerization vessel without structural damage to the large area, thin walls of low-density material.

A still further object is to provide for the elimination of the creation of an explosive atmosphere and exposure of operating personnel to toxic fumes during the manufacturing of wood-plastic composites by eliminating the necessity for handling the wood from the time it is placed into the impregnation-polymerization vessel until the processing is completed.

Another major object is to provide a novel apparatus for loading the monomer into the several pieces of wood that are inside a container and thereafter removing from the container any excess monomer.

Still another major object is the provision of a novel apparatus for effecting polymerization of the monomer impregnated in the wood by radiation from a source submerged in a pool of water thereby reducing the irradiation hazards to operating personnel. This apparatus and method involve unique container-handling equipment for subjecting the monomer impregnated wood to irradiation that is substantially uniform in duration and intensity.

Other objects and advantages will become apparent from the claims and from the following description when read in conjunction with the appended drawings.

THE DRAWINGS FIG. 1 is a flow chart illustrating the basic conversion process;

FIG. 2 is a plan view of the novel canister of the instant invention with the top removed;

FIG. 3 is a side view of the canister of FIG. 2;

FIG. 4 is an exploded view showing the details of the means encircled in FIG. 3 for fastening the top to the canister;

FIG. 5a is a diagrammatic view of the system used at the impregnation station;

FIG. 5b is a front view of the apparatus for supporting the canister of FIGS. 1 and 2 during the pressure impregnation of the wood to be converted;

FIG. 6 is a partially cut away plan vie of the apparatus of FIG. 5b with the monomer and inert gas storage drums removed;

FIG. 7 (sheet 2) is an exploded view of the means for moving the canister supports of FIG. 5b and 6 together;

FIG. 8 is a perspective view of an oven suitable for use in the heat conversion process;

FIG. 9 is a schematic side view of the station where a source of radiation is submerged in a pool of water and the canisters loaded with monomer impregnated wood are lowered in radiation receiving proximity thereto to effect the curing of polymerization of the monomer;

FIG. 10 is a plan view of the station shown in FIG. 9 to illustrate the rails for suspending the radiation source and canisters in the water pool; and

FIG. 11 is a side view illustrating the location of the radiation source between two canisters loaded with plastic impregnated wood and the presence of gas bubbles in the water to improve the effectiveness of irradiation.

DESCRIPTION OF THE SYSTEM The system of the present invention is illustrated by the flow chart of FIG. 1. Wood from a convenient storage area is prepared, e.g., by sawing or other machining to size, for loading into canisters which have been cleaned from previous operations. Once the canister has been loaded with the wood to be converted to wood-plastic composite, the canister is sealed and transported to the impregnation station. At the impregnation station, air is evacuated from within the canister and to some extent from the void spaces between the fibers within the wood. The resulting vacuum is then released by an inert gas such as nitrogen. The inert gas is in turn evacuated from within the canister and an impregnating fluid, such as methylmethacrylate, together with a catalyst, if desired, is pumped into the canister for positive pressure impregnation .by the fluid into the body of the wood. After a sufficient time has elapsed to allow the desired degree of impregnation, the excess fluid, that is the fluid unabsorbed by the wood, is drained from the canister under a cover of the inert gas.

After impregnation of the wood, the canister is transported to the conversion station where the monomer is polymerized as by irradiation with gamma rays or by the application of heat in the presence of a catalyst. After the polymerization of the fluid monomer within the wood, which converts the wood to wood-plastic composite, the canister is transported to the unloading station where the wood-plastic composite is removed for further processing, such as sanding and polishing, as may be desired. Any polymerized excess monomer remaining in the canister is removed and the canister is otherwise cleaned and prepared for reuse.

DESCRIPTION OF CAN ISTER Referring now to FIGS. 2, 3 and 4 for the details of canister construction, the main body 10 is illustrated to have a generally rectangular boxlike configuration. Bottom 11 is thin and flat. Sidewalls l2 and 14 and end walls 16 an 18 are comparatively thick and rigid, and terminate at the upper end in a flange 20. Top 22 is similarly constructed having side and end walls of a depth sufficient only to provide a flange 24 which mates with the flange 20 of the sidewalls 12 and 14 and end walls 16 and 18 around the entire periphery of the canister. Although the canister 10 may be of any convenient size, internal dimensions of slightly more than 4 feet by 8 feet have been found desirable to accommodate commercially available wood products. Canister depth requirements are dictated in part by gamma ray absorption and are generally less than 20 inches. In actual practice 12-inch depth has been found suitable and a smaller canister having internal dimensions of 2 feet by 4 feet by 8 inches has been found convenient for many operations.

End wall 16 may be equipped with handles or hooks 28 to which means for handling the canister may be attached. End wall 16 may also be equipped with a handle 30 which is particularly adapted for use in the hanging of the loaded canister in radiation shielding medium during the irradiation polymerization process. Handles 28 and 30 may, of course, be combined into any single attachment means whereby the handling of the canister can be facilitated.

End wall 18, when the canister is hung by handles 28 and 30, becomes a bottom wall. It therefore is shaped at a point approximately midway the extremities thereof to form a very shallow V. An externally operated valve 26 may be inserted at the apex of the V to facilitate drainage of the canister.

Externally operated valves 32 and 34 may be located respectively immediately adjacent the sidewalls 12 and 14 at opposite ends of end wall 16. Their functions are described below in connection with F 165. b and 6.

Since the canister must be airtight, a seal is provided between main body of the canister and the top member 22. A groove 36 may be cut into the upper surface of flange to receive a gasket 38. Gasket 38 is generally tubular in cross section and is of a substance not readily copolymen'zed such as neoprene. It has been found beneficial to coat the gasket 38 with a petroleum jelly such as vasoline prior to each use of the canister 10. Gasket 38 will eventually harden or graft copolymerize with monomer under exposure to gamma rays and must be periodically inspected and replaced Complete separation of top member 22 from the main body 10 of the canister is desirable for ease of loading. Quickrelease mechanisms may be advantageously used to secure top 22 to main body 10, though mechanisms having a tendency to corrode or otherwise become inoperable when submerged in water should be avoided. A satisfactory fastener may consist of threaded bolts 40 and nuts 42 secured to the lower side of flange 24 as shownin FIG. 4. Nuts 42 may be secured on the lower side of flange 24 as by plate 44 and screws 46 or by welding if desired.

Pockets 48 may be provided along the external sidewalls 12 and 14 of the canister 10 for the purpose of receiving weights to be added as necessary to assist in the submergence of the loaded canister within a radiation absorbing or shielding fluid in the irradiation conversion process and to compensate for shifts in the center of gravity which may accompany the uneven loading of the canister 10 with the wood to be converted.

The selection of a material out of which to construct the single, multifunction canister of the present invention is subjected by conflicting considerations. It is essential that the material be of sufficient strength to withstand high internal pressures during the time when the monomer impregnation takes place and external pressures when submerged; that it have high resistance to corrosion while submerged in a radiation absorbing medium such as water; and that it have low gamma ray absorption characteristics, meaning it should have a low density and be as thin as possible on the large area surfaces through which the radiation passes. Aluminum has been found to the preferred material for the thin, flat radiation receiving surfaces of bottom 11 and top 22. The sidewalls may be of other materials, or made sufficiently strong by increasing the dimensions thereof.

Water is liberated from the wood during the conversion process. In those applications where the canister is submerged and hence cooled, and to a lesser extent where the polymerization occurs in an oven, the liberated water and some monomer has been observed to condense upon the internal wall surfaces of the canister 10.

Contact of the product with water results in discoloration. To reduce or prevent discoloration, a liner 49 of an insulative material such as cardboard may be inserted in the canister as illustrated in FIG. 2. Liner 49 is spaced slightly from the internal walls by small blocks 51. Water and monomer condensing on the internal surfaces of the canister 10 behind the liner 49 drain downwardly to collect in the slight V in the end wall 18. The polymer may be removed from the canister 10 after the canister is removed from the polymerization station.

Depending upon the depth of submergence of the canister in the radiation pool of FIGS. 11-13 and the internal canister pressure that is applied, the thin, flat top 22 and bottom 11 of the canister 10 may require internal support to prevent buckling inwardly. This support may be supplied by the wood products themselves or several supports of the proper thickness may be inserted between the top and bottom surfaces in the canister 10 prior to the sealing thereof. Pressurization of the canister with an inert gas after impregnation aids in reducing the pressure differential as the canister is submerged. However, the polymerization reaction is exothermic, and the increase in internal temperature will produce an increase in the internal pressure. Therefore, any positive pressure applied to the canister prior to the polymerization step should be determined by taking into consideration the maximum internal temperature that can be anticipated.

There is a similar problem of internal pressure that must be considered at the impregnation station. Buckling outwardly of flat top 22 and bottom 11 is prevented when the internal pressure of impregnation is applied by the utilization of the novel external supports to be described next in connection with FIGS. 5b and 6.

Once the canister has been loaded with wood to be converted and the top 22 secured thereto by means of bolts 40 and nuts 42, the loaded canister is transported by any convenient means such as an overhead rail and trolley system to the impregnation station shown in FIGS. 5b and 6. There it may be turned so as to rest upon sidewall 14 and placed in the impregnation apparatus. Orientation of the canister at this station with respect to the impregnation apparatus dictates the location of certain canister valves as will be hereinafter explained.

DESCRIPTION OF FLUID IMPREGNATING STATION Referring now to FIG. 5a, the apparatus used for effecting the impregnation process is illustrated in diagrammatic form. Air may be evacuated from canister by closing valve 34 and operating pump 90 which is connected to valve 32 through valve 103, cold trap 87a, surge tank 89, valve 100 and line 98. The vacuum within the canister 10 is thereafter released with the inert gas from drum 88 through line 96, valve 108 and line 98. Valves 100 and 104 are at the moment closed.

The inert gas may be fully or partially evacuated from the canister 10 by closing valve I08 and again connecting pump 90 to line 98 and to valve 32 by opening valves I00 and 103, after evacuation of canister 10 then pump 90 is shut off. Monomer from drum 86 is then introduced into canister 10 through lines 94 and 99 into opened valve 34 of canister 10 by pump 92. Surge tank 89 maintains a vacuum condition within the canister 10 until the monomer level reaches valve 32 at which time valve 32 is closed. It has been found that the impregnation process is accelerated if the canister 10 is entirely filled with monomer and pressurized by application of inert cover gas or compressed air up to about 150 lbs./in. the optimum pressure depending upon the impregnating fluid viscosity and the porosity of the wood to be impregnated. Supplying the monomer at a pressure to about 40 lbs./in is satisfactory for many applications.

Once sufficient time has elapsed for the wood within the canister 10 to reach the desired state of impregnation, pump 92 is used with flow reversed to assist the draining of unabsorbed monomer from the canister through line 99, pump 92, and line 94 to the monomer tank 86.

It is, of course, necessary to remove the unabsorbed monomer from within the canister 10 prior to the polymerization thereof to prevent polymerization of the entire contents of the canister 10 in a single, homogeneous mass. The monomer collected in surge tank 89 may be subsequently transferred to drum 86 for reuse.

It is to be recognized that numerous variations in the process cycle are possible which will affect the concentration and distribution of monomer within the wood. The parameters having greatest effect are the degree of treating vacuum in the second vacuum stage, the inert gas pressure applied at its time 'duration after draining off the excess monomer, the soaking pressure and time and the viscosity of the impregnating fluid.

Variations are possible in the impregnation process to permit the attainment of uniform partial monomer loadings and also monomer loadings tending to localize the monomer near the surface. Such partial loadings, when achieved under controlled conditions, are of economic importance since they reduce the amount, and hence cost, of the monomer without reducing the desired properties of the finished product for many applications,

These variations are achieved by varying primarily the degree of the vacuum and the applied pressures. If a partial vacuum only is used just prior to the filling of the canister with the liquid monomer, thereafter when the pressure is applied the entrapped nitrogen in the wood parts will tend to hold back the monomer and create a higher concentration of monomer near the surface.

The impregnating apparatus illustrated in FIGS. 5b and 6 may comprise a rigid frame of a generally boxlike construction which may be any suitable structural material such as carbon steel. Rotatably mounted on spaced, parallel structural members 50 extending across the base members 52 of the frame in a direction perpendicular thereto are a plurality of spaced, parallel rollers 54 which together comprise the surface upon which the sidewall 14 of canister 10 rests.

Extending between the base members 52 and the top members 56 of the frame in a vertical plane is a rigid fixed support plane 58 which is permanently secured thereto as by welding. One of the large thin walled surfaces 11 of the canister is illustrated as abutting support plate 58 which supports the canister wall during the impregnation operation.

A pair of rails 60 are mounted horizontally between the fixed support plate 58 and a fixed structural member 62 which extends parallel to support plate 58 along each of the rails 60. Suspended from the trolleys 64 in a plane substantially parallel to that of fixed support plate 58 is a movable support plate 66 that is of a size similar to that of support plate 58 and the largest canister to be accommodated at the loading station.

The loaded canister 10 is illustrated as having been turned up so that sidewall 14 rests on rollers 54. In this position the thin, flat bottom wall 11 and top wall 22 are supported respectively by the fixed and movable support plates 58 and 66. In the actual impregnation operation it has been found convenient to place two or more canisters in side-by-side relationship between the fixed and movable support plates 58 and 66, the adjacent tops 22 and bottoms 11 of the respective canisters 10 providing mutual support for each other against expansion due to the internal impregnation pressures.

A plurality of rods 68 extend through aligned apertures in the fixed and movable support plates 58 and 66 above and below the canister 10. Rods 68 are supported at the extreme left by horizontal structural members 62 and 70, the rods rotatably extending through circular apertures therein. The extreme left ends 72 of the rods 68 may be machined to have a hexagonal cross section so that they may be easily gripped for rotation.

The extreme right ends 74 of the rods 68, i.e., the ends extending through the fixed support 58, are threaded through a nut 76 secured to the fixed support 58. Intermediate the hexagonally machined ends 72 and the threaded ends 74, the rods 68 may have grooves 78 at spaced intervals to receive a keeper 80 which is substantially wider than the aperture in the movable support 66.

The canisters are supported between the fixed and movable support plates 58 and 66 for pressure impregnation in the following manner. Trolley 64 is used to position movable support 66 in proximity to the top 22 of the canister 10. Keepers 80 are then inserted in the grooves nearest to the external surface of movable support 66 and rods 68 are rotated by the application of rotatable means such as hand crank 82. Inasmuch as nut 76 has been secured to the fixed support 58, rotation of the rods 68 will cause keeper 80 to be drawn up against the rear of movable support 66 moving it towards the fixed support 58 and exerting pressure on the top 22 and bottom 11 of the canister 10.

Mounted for convenience at the top of the frame upon arcuate supports 84 are bottles or drums 86 and 88 containing respectively a suitable impregnating fluid including monomer additives with or without catalyst and an inert gas such as nitrogen. Fluid communication between the drums 86 and 88 and the inputs to pumps 90 and 92 may be achieved by means of quick disconnect lines 94 and 96 respectively. Fluid lines 98 and 99, also utilizing connectors of the quick disconnect type, provide alternative fluid communication between pump inlet/outlet valves 10% and 102 respectively of surge tank 89 and pump 92 and valves 32 and 34 of the canister 10.

After impregnation, provision may be made for tilting the canister to facilitate draining. Alternatively, or in addition, the canister may be hung from hooks 28 for a brief period. Excess monomer collects in the V of canister end wall 18 and may be removed as by means of valve 26. The addition of an inert gas into valves 32 or 34 aids in the draining process by preventing the creation of a vacuum in the canister.

One important advantage of the present invention is that the wood is not handled after impregnation and prior to polymerization. Because many monomers are toxic, it is important for commercial operation that the canister configuration be adapted not only for the impregnation operation, but also for the polymerization operation.

In addition, monomer may condense on the internal walls of the canister during the polymerization operation and collect beneath support 87 in the V of canister end wall 18. Polymerization of this monomer may be retarded by the use of lead support 87 where irradiation is used or by the introduction of a solvent containing a concentrated inhibitor added through valve 26 prior to starting the polymerization step.

CURING BY HEAT Turning now to the polymerization of the monomer impregnated wood, the process may be accomplished by the application of heat, by irradiation as with gamma rays, or a combination of both heat and irradiation. A catalyst is conventionally added in solution with the monomer prior to impregnation of the wood. The catalyst decays when heated to produce the free radicals necessary to polymerize the monomer.

An oven suitable for the heat polymerization process is illustrated in FIG. 8 wherein the insulated walls 116 are shown interiorly lined with a serpentine array 118 of pipes in fluid communication with a source of steam or hot air (not shown). Alternative sources of heat such as electrical heating elements may be used if desired.

A plurality of spaced, parallel rollers 120 are illustrated as being rotatably mounted between supports 122 secured to the lower internal surface 124 of the oven to facilitate the moving of the canisters 10 into and out of the oven. The upper surface of these rollers together comprise the supporting surface for the canisters 10 placed in the oven.

Other oven configurations may be substituted for the embodiment shown without departing from the scope of the present invention. Similarly, other means for heating the oven may be employed in lieu of stem. Additional arrays of steam carrying pipes may be interspersed between the canisters in the event that it is desirable to heat a large number of canisters simultaneously.

CURING BY COMBINED TECHNIQUES It has been found advantageous to use catalysts with monomer when polymerization is initiated by radiation. In this case, depending upon the available radiation intensity, one may choose to use mixed catalysts having different rates of decomposition. A small amount of short, half-life catalyst at temperatures of 90 to 120 F. will aid in overcoming inhibitors and reduce the time to the gel-stage. In addition, a small amount of short, half-life catalyst at temperatures reached during the exotherm will produce free radicals tending to aid in termination of polymer chains.

Similarly in heat curing, radiation can also be used to achieve a more complete cure. The addition of a free radical producing catalyst to the monomer is not necessary in the radiation polymerization process. The process is exothermic and substantially self-propagating once the gel stage is reached. The probability for continued molecular chain growth decreases, however, with the reduction in the number of irradiation produced free radicals. For this reason, it is deemed preferable to irradiate the product after the exotherm in order to more fully complete the polymerization of the monomer.

CURING BY RADIATION Instead of (or in addition to) using the heated oven of FIG. 8 to effect the curing of the monomer, the impregnated wood filled canisters 10 may be transported to an irradiation tank as by means of a conventional rail and trolley system shown schematically in FIG. 9. The tank itself may be of an configuration but it is here illustrated to have a generally cylindrical shape. The irradiation station extends for some distance both above and below the surface of the earth in which it is placed. The upper portion 122 serves as a weather shield and is covered with a roof 124 having a closable opening 126 therein through which a lead transfer cask containing the individual source elements is lowered into the pool. The cask can then be safely unloaded under water and the source elements remotely handled and placed into the source plaque.

Source plague 128 may be substantially flat and have a plurality of cobalt slugs 130 at spaced intervals along the length thereof. The plaque 128 is slightly less than one-half the 4 foot width of the larger canisters and is sufficiently longer than the 8 foot length of canisters 10 to provide uniform exposure despite the end effects. Positioning the plaque 128 adjacent first one and then the other extreme edges of the canisters while maintaining the canisters stationary results in the exposure of the center section of the canister 10 to a reduced dosage but for a longer period of time, the time integral of the radiation received being determinative in the conversion.

A rail and trolley system 132A for handling the cask (not shown) for the source elements 130 and for removing the canisters 10 from the transportation rail 132 and for lowering them into the tank may be supported by rails 133 in the portion 122 of the irradiation station that extends above ground.

The tank is filled slightly below ground level with a radiation absorbing fluid 134 such as water. Suspended by channels 135 and rods 136 beneath the surface 138 of the fluid 134 is a rail and trolley array shown in greater detail in FIG. 10. The depth at which the array is suspended is material only to the extent that radiation hazards to operating personnel are eliminated at the surface 138 of the fluid 134.

Rigidly attached to the lower surface of a pair of spacedparallel supporting members 140 are three parallel canister rails 142, 144 and 146. Each canister rail is equipped with a pair of trolleys 148 operable, one operable over mutually exclusive portions of the canister rails. Suitable pneumatic or hydraulic cylinders 150 and pistons may be provided to independently position the trolleys 148, the means 152 for controlling the position of the trolleys 148 being remotely located above the surface 138 of the fluid 134.

The source plaque 128 is suspended from rail 154 by means of trolley 158 and a retractable piston 159. The lower end 160 of plaque 128 rides in guide 162 restricting movement back I and forth along a single vertical plane. Guide 162 is of sufficient depth to allow approximately 6 inches vertical displacement of the plaque 128 as it is raised and lowered by piston 159 in irradiating the canisters 10.

The vertical movement of the source plaque 128 is desirable to smooth out variations in the radiation pattern which may result from the use of line sources 130.

The position of plaque 128 supporting trolley 158 along rail 154 and the position of piston 159 are preferably controlled remotely from a control station 152.

As earlier mentioned, the canisters 10 may conveniently be made in two sizes, the large substantially twice the dimensions of the smaller. In the illustrated embodiment only two of the larger canisters 10 may be simultaneously suspended from the array, one from each end of the center canister rail 144. Rail 144 is not centered with respect to the tank but is offset sufficiently so that it is centered with respect to the space remaining between the radially innermost end of the plaque 128 when positioned at the extreme end 164 of rail 154 and the opposite wall 166 of the tank. This allows the canisters to be rotated by rotary actuators 157 so that opposite sides of the canister which have the thin, comparatively radiation transparent walls to be sequentially exposed to the radiation plaque 128.

The lower end 160 of plaque 128 is of hollow construction, is apertured, and is in fluid communication as by way of flexible hose 168 with a source 170 of an inert, nonradiation absorbent gas such as nitrogen. A plenum chamber 172, the lower surface 174 of which is positioned slightly beneath the surface 138 of the radiation absorbing fluid 134 is used to collect water and the inert gas pumped under pressure by pump 176 from the source 3170 to be released at the plaque 128. The mixture of inert gas and radiation absorbing fluid drawn from the plenum chamber 172 by means of line 178 and pump 176 is fed to a separator 180. The liquid from separator 180 may be returned to the tank after appropriate filtering. By introducing the recirculated water to pass along the surface of plaque 128, the water is continually being irradiated and the growth of bacteria is arrested. The inert gas separated is compressed and stored at 170 for recirculation.

Referring now to FIG. 11, the compressed bubbles of inert gas are shown to be released near the bottom of the tank where they expand as they rise to the surface 138 of the pool. The region of release at the bottom of the pool is between the substantially parallel sides of the source plaque 128 and of the canisters that are suspended from the canister rails 142, 144 and 146 in radiation receiving proximity thereto. In displacing the radiation absorbing fluid or water 134 from this space, the presence of the inert gas decreases the absorption of radiation energy from the source plaque 128. The bubble stream further creates a region of reduced pressure which causes the canisters 10 to swing into closer proximity to the source plaque 128 without the danger of causing a collision. The means 180 by which the canisters 10 are suspended are constructed as by a pin and socket arrangement so as to allow only a single degree of movement. Motion of the canisters 10 is thus possible only toward and away from the source plaque 128.

The term wood" has been used throughout more in the generic sense. It should be recognized that this application refers to cellulose materials in general which include wood in lumber and shaped forms, hardboard or particleboard which is a mixture of wood chips and adhesives, paper and paperboard. All these materials are successfully converted in the process and equipment comprising the invention.

Many modifications will appear to those skilled in the art to which this invention pertains. It is to be therefore understood that the scope of the invention is not to be limited to the embodiments disclosed, but is rather to be restricted only by the language of the appended claims and the range of equivalents to be accorded thereto.

What I claim is:

1. Apparatus for converting wood to a wood-plastic composite comprising:

a. a canister having a pair of substantially planar walls which are insufficiently thick to withstand internal pressures substantially in excess of atmospheric pressure and an openable side for insertion of the wood to be converted;

b. means for closing the operable sides and forming a sealed canister;

. means for evacuating the air from within said canister;

. means for filling said canister with an inert gas;

. an impregnating station including means for evacuating the inert gas from within said canister, means for impregnating wood inside said canister with a monomer under a pressure substantially in excess of atmospheric pressure; and means for externally supporting said planar canister walls against separation when superatmospheric pressures are applied to the interior of said canister; means for removing the excess monomer from within said canister; and

g. means for polymerizing the monomer within the wood while the wood remains in said canister.

2. The canister of claim 1 further including means for spacing the wood from interior surfaces of the canister walls to prevent condensation which forms on said interior surfaces from contacting said wood.

3. The apparatus of claim 1, wherein said polymerizing means comprises a source of gamma radiation, said parallel canister walls are sufficiently thin to pass radiation for effecting polymerization of said monomer, and said impregnating station includes means for supplying a monomer under a pressure of at least 40 pounds per square inch.

4. The apparatus of claim 3 wherein said means for polymerizing the monomer includes a source of gamma radiation sufficiently submerged within a radiation absorbing fluid to provide biological protection, and means for submerging said canister into said fluid to be in radiation receiving proximity with respect to said source.

5. The apparatus of claim 3 wherein said impregnating station comprises;

a. a frame;

b. a first support member mounted to said frame and adapted to abut one of said canister walls;

c. a second support member mounted to said frame and adapted to abut the other of said canister walls; and

d. means for moving said first and second support members toward and away from each other to permit insertion and removal of said canister at said impregnating station.

6. The apparatus of claim 5 wherein said impregnating station comprises means for supporting the canister between said supports including a plurality of spaced parallel rollers mounted to said frame and having axes disposed in the direction of movement of said support members.

7. The apparatus of claim 5 wherein said first support member is rigidly secured to said frame and said support member is mounted for movement relative to said first support member by means including spaced parallel rails secured to said frame, a trolley movable along each of said rails, and means for suspending said second support member from said trolley.

8. The apparatus of claim 7 wherein said means for moving said movable support includes a plurality of rods threaded through nuts fixed relative to said frame, a keeper on said rods adapted to engage said second support member, and means for rotating said rods to urge said second support member toward said first support member.

9. The apparatus of claim 8 wherein the impregnating station further includes means for supporting the canister between said supports comprising a plurality of spaced-parallel rollers mounted to said frame and having axes disposed in the direction of movement of said second support member whereby the canisters are inserted and moved along said rollers and secured in position by movement of said second support member against one of said canister walls.

10. The apparatus of claim 1 wherein said canister comprises; rigid, upstanding side and end walls of a strength sufficient to withstand the internal pressure of impregnating the wood with the monomer; wherein said parallel canister walls comprise a bottom wall coextensive with said upstanding side and end walls and secure thereto and a top wall coextensive with said upstanding side and end walls, said top and bottom walls being substantially parallel and sufficiently thin to permit polymerization of the monomers by exposure to radiation, and all ofsaid top wall being removable to provide access to the interior of the canister for loading and unloading; and wherein said canister further comprises a seal disposed between said top wall and said side and end walls; and means for releasably securing said top wall to said upstanding side and end walls.

11. The apparatus of claim 10 wherein said impregnating station further includes a. a frame;

b. fixed and movable support members attached to said frame for supporting the top and bottom walls of the canister;

c. means for supporting the canister between said fixed and movable support members;

d. means for moving said movable support member toward said fixed support member to support the thin walls of the canister;

e. a supply source of a monomer to be impregnated in said wood;

f. a source of an inert gas;

g. pumping means for selectively creating a positive or negative pressure within the canister; and

h. means for selectively connecting said sources of monomer and inert gas to said pumping means. 12. The apparatus of claim 11 wherein one of said end walls is provided with an externally operated valve for draining the interior of the canister and wherein the other of said end walls is provided with attachment means for moving the canister.

13. The apparatus of claim 12 wherein the other one of said end walls has externally operated valve located adjacent each of said sidewalls for introducing and removing the inert gas and monomer from said canister and including a lining of insulating material positioned adjacent the interior walls of the canister so as to prevent the contact of the wood to be converted with said interior wall surfaces.

14. The canister of claim 13 wherein said means for releasably securing said top wall to said upstanding side and end walls includes a plurality of threaded bolts secured through apertures in the cooperating surfaces of said side and end walls externally of said seal.

15. The apparatus of claim 1 wherein the means for polymerizing the monomer comprises:

a. a tank having bottom and upstanding walls filled with radiation absorbing fluid;

b. means for supporting a source of radiation within said radiation absorbing fluid; and

c. means for lowering the canister containing wood to be converted into said fluid and into radiation receiving proximity of said source and for raising said canister after irradiation.

16. The apparatus of claim 15 further comprising means for removing part of the radiation absorbing fluid between the radiation source and the canister which includes means for releasing an inert gas into said fluid between the canister and said source at the lower end thereof.

17. The apparatus of claim 13 further comprising means for recirculating said inert gas which includes a. a plenum chamber for collecting said gas as it rises in said fluid from between the canister and said source;

b. pump means for evacuating said plenum chamber;

c. separating means for separating the fluid from the gas evacuated from said plenum chamber;

d. means for storing the separated gas; and

e. means for pumping said separated gas to the lower end of said source.

18. The apparatus of claim 15 further including means for rotating and for moving said canister which includes:

a. a rail supported beneath the surface of said fluid;

b. a trolley movable along said rail;

c. means for moving said trolley along said rail;

d. support means for rotatably supporting said canister from said trolley; and

e. means for rotating said support means.

19. The irradiation pool of claim 18 wherein said means for rotating and moving said canister includes:

a. a plurality of rails supported within said fluid in a spacedparallel relationship;

b. two trolleys mounted for movement along mutually exclusive portions of each of said rails;

c. hydraulic means for moving said trolleys along said rails;

d. means for supporting a canister from each of said trolleys,

said means including a vertical shaft; and

e. means for independently rotating the shaft of each of said support means.

20. The apparatus of claim 15 wherein said source is an elongated flat plaque that is movable across said tank by means including:

a. a rail substantially traversing the width of said tank and supported beneath the surface of said fluid;

b. a trolley mounted on said rail;

0. means for supporting said plaque from said trolley;

d. means for moving said trolley along said rail; and

e. means for restraining said plaque against upward and downward movement during normal operation of said apparatus. 

1. Apparatus for converting wood to a wood-plastic composite comprising: a. a canister having a pair of substantially planar walls which are insufficiently thick to withstand internal pressures substantially in excess of atmospheric pressure and an openable side for insertion of the wood to be converted; b. means for closing the operable sides and forming a sealed canister; c. means for evacuating the air from within said canister; d. means for filling said canister with an inert gas; e. an impregnating station including means for evacuating the inert gas from within said canister, means for impregnating wood inside said canister with a monomer under a pressure substantially in excess of atmospheric pressure; and means for externally supporting said planar canister walls against separation when superatmospheric pressures are applied to the interior of said canister; f. means for removing the excess monomer from within said canister; and g. means for polymerizing the monomer within the wood while the wood remains in said canister.
 2. The canister of claim 1 further including means for spacing the wood from interior surfaces of the canister walls to prevent condensation which forms on said interior surfaces from contacting said wood.
 3. The apparatus of claim 1, wherein said polymerizing means comprises a source of gamma radiation, said parallel canister walls are sufficiently thin to pass radiation for effecting polymerization of said monomer, and said impregnating station includes means for supplying a monomer under a pressure of at least 40 pounds per square inch.
 4. The apparatus of claim 3 wherein said means for polymerizing the monomer includes a source of gamma radiation sufficiently submerged within a radiation absorbing fluid to provide biological protection, and means for submerging said canister into said fluid to be in radiation receiving proximity with respect to said source.
 5. The apparatus of claim 3 wherein said impregnating station comprises; a. a frame; b. a first support member mounted to said frame and adapted to abut one of said canister walls; c. a second support member mounted to said frame and adapted to abut the other of said canister walls; and d. means for moving said first and second support members toward and away from each other to permit insertion and removal of said canister at said impregnating station.
 6. The apparatus of claim 5 wherein said impregnating station comprises means for supporting the canister between said supports including a plurality of spaced parallel rollers mounted to said frame and having axes disposed in the direction of movement of said support members.
 7. The apparatus of claim 5 wherein said first support member is rigidly secured to said frame and said support member is mounted for movement relative to said first support member by means including spaced parallel rails secured to said frame, a trolley movable alOng each of said rails, and means for suspending said second support member from said trolley.
 8. The apparatus of claim 7 wherein said means for moving said movable support includes a plurality of rods threaded through nuts fixed relative to said frame, a keeper on said rods adapted to engage said second support member, and means for rotating said rods to urge said second support member toward said first support member.
 9. The apparatus of claim 8 wherein the impregnating station further includes means for supporting the canister between said supports comprising a plurality of spaced-parallel rollers mounted to said frame and having axes disposed in the direction of movement of said second support member whereby the canisters are inserted and moved along said rollers and secured in position by movement of said second support member against one of said canister walls.
 10. The apparatus of claim 1 wherein said canister comprises; rigid, upstanding side and end walls of a strength sufficient to withstand the internal pressure of impregnating the wood with the monomer; wherein said parallel canister walls comprise a bottom wall coextensive with said upstanding side and end walls and secure thereto and a top wall coextensive with said upstanding side and end walls, said top and bottom walls being substantially parallel and sufficiently thin to permit polymerization of the monomers by exposure to radiation, and all of said top wall being removable to provide access to the interior of the canister for loading and unloading; and wherein said canister further comprises a seal disposed between said top wall and said side and end walls; and means for releasably securing said top wall to said upstanding side and end walls.
 11. The apparatus of claim 10 wherein said impregnating station further includes a. a frame; b. fixed and movable support members attached to said frame for supporting the top and bottom walls of the canister; c. means for supporting the canister between said fixed and movable support members; d. means for moving said movable support member toward said fixed support member to support the thin walls of the canister; e. a supply source of a monomer to be impregnated in said wood; f. a source of an inert gas; g. pumping means for selectively creating a positive or negative pressure within the canister; and h. means for selectively connecting said sources of monomer and inert gas to said pumping means.
 12. The apparatus of claim 11 wherein one of said end walls is provided with an externally operated valve for draining the interior of the canister and wherein the other of said end walls is provided with attachment means for moving the canister.
 13. The apparatus of claim 12 wherein the other one of said end walls has externally operated valve located adjacent each of said sidewalls for introducing and removing the inert gas and monomer from said canister and including a lining of insulating material positioned adjacent the interior walls of the canister so as to prevent the contact of the wood to be converted with said interior wall surfaces.
 14. The canister of claim 13 wherein said means for releasably securing said top wall to said upstanding side and end walls includes a plurality of threaded bolts secured through apertures in the cooperating surfaces of said side and end walls externally of said seal.
 15. The apparatus of claim 1 wherein the means for polymerizing the monomer comprises: a. a tank having bottom and upstanding walls filled with radiation absorbing fluid; b. means for supporting a source of radiation within said radiation absorbing fluid; and c. means for lowering the canister containing wood to be converted into said fluid and into radiation receiving proximity of said source and for raising said canister after irradiation.
 16. The apparatus of claim 15 further comprising means for removing part of the radiation absorbing fluid between the radiation source And the canister which includes means for releasing an inert gas into said fluid between the canister and said source at the lower end thereof.
 17. The apparatus of claim 13 further comprising means for recirculating said inert gas which includes a. a plenum chamber for collecting said gas as it rises in said fluid from between the canister and said source; b. pump means for evacuating said plenum chamber; c. separating means for separating the fluid from the gas evacuated from said plenum chamber; d. means for storing the separated gas; and e. means for pumping said separated gas to the lower end of said source.
 18. The apparatus of claim 15 further including means for rotating and for moving said canister which includes: a. a rail supported beneath the surface of said fluid; b. a trolley movable along said rail; c. means for moving said trolley along said rail; d. support means for rotatably supporting said canister from said trolley; and e. means for rotating said support means.
 19. The irradiation pool of claim 18 wherein said means for rotating and moving said canister includes: a. a plurality of rails supported within said fluid in a spaced-parallel relationship; b. two trolleys mounted for movement along mutually exclusive portions of each of said rails; c. hydraulic means for moving said trolleys along said rails; d. means for supporting a canister from each of said trolleys, said means including a vertical shaft; and e. means for independently rotating the shaft of each of said support means.
 20. The apparatus of claim 15 wherein said source is an elongated flat plaque that is movable across said tank by means including: a. a rail substantially traversing the width of said tank and supported beneath the surface of said fluid; b. a trolley mounted on said rail; c. means for supporting said plaque from said trolley; d. means for moving said trolley along said rail; and e. means for restraining said plaque against upward and downward movement during normal operation of said apparatus. 